1-Firstly, go the your profile picture at the extreme right corner of the site near the Bell notification and Messages notification
2-Click on Setting on red button
3-Log in first by typing your Username and Password
4-They are 4 options in the Account Settings,
Click on 2nd option i.e Update Profile and scroll down until you see 'Current Avatar'
6-Now click on 'Upload your Own'
7- You will see 'No file chosen'. Now click on 'Choose File' and make sure the picture you want to upload is a PNG format
8-Finally, scroll all the way down and hit 'Update'